US2425047A - Frequency responsive control system - Google Patents

Description

1947- J. B. REEVES FREQUENCY RESPONSIVE CONTROL SYSTEM Filed NOV. 20, 1944 n q d a SLIP FREQUENCY Patented Aug. 5,1947

FREQUENCY RESPONSIVE CONTROL SYSTEM James'BlReeves, Whitefish Bay, Wls., assignor to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application November 20, 1944, Serial No. 564,356

The invention relates to a control system which is responsive to a current of variable frequency and while not limited thereto is particularly applicable for controlling the starting and operation 9 Claims. (Cl. 172-289) the alternating current power supply, it is essential that the field winding energizing current be initially supplied at a moment at which the field poles bear a certain space relation to the rotating of synchronous motors. alternating magnetic field induced by the alter- In starting synchronous motors it is desirable nating current. The present system assures such to supply energy from an alternating current power relation and further assures that if the source to the armature of the motor, while the motor after being synchronized should be pulled field winding is short circuited upon itself. When out of step due to overload or other causes, the the motor has reached nearly synchronous speed, ill direct current excitation is reduced and is again the short circuit is removed from the field winding brought up to its normal value and the motor is and the latter is connected to a supply of direct re-synchronized when the disturbance has discurrent, whereupon the motor pulls into step. It appeared. is also desirable to arrange for disconnection of Referring to the drawing, the same illustrates the direct current supply and for short circuiting bus lbars Ll, L2, and L3, which supply three-phase of the field winding upon the motor pulling out alternating current energy through a manually of step due to an overload or any other cause, so operated switch 2 to the primary winding 3 of a that it may be synchronized again in the usual synchronous motor 3. The motor is also provided manner without interrupting the supply of alterwith a field winding 3. nating current to the armature. so When the switch 2 is closed, it also energizes The present invention has for one of its objects the auxiliary bus bars Li I and LIZ to the potento provide a system for synchronizing synchrotial of the bus bars LI and L2, respectively, to nous motors. supply a single phase voltage to the primary wind- Another object is to provide in a synchronous ing i of a transformer 4, which is also provided motor starter means for controlling the direct 26 with a center tapped secondary winding 4 The current excitation of the field winding of the transformer winding 4 supplies alternating curmotor. rent which is rectified to energize the winding 3 Another object is to provide means aflording a Connected to the end terminals of the winding control voltage which is a function of a variable 4 are the anodes 5 and 6, respectively, of gaseirequency. 39 ous electron tubes 5 and 6. These tubes are also Another obiectlsto provide means for chargin provided with cathodes 5 and B, respectively, a condenser at a given rate and discharging it and control electrodes 5 and 6, respectively. The during the operation of the charging means at cathodes 5 and 6 are jointly connected to one intervals varying in frequency. terminal of the winding 3, the other terminal of Another object is to provide means responsive the said winding being connected to the center to a voltage which varies as a function oflthe tap of the winding i The control electrode 5 ratio of two frequencies. is connected to one end terminal and the control Other objects and advantages will hereinafter electrode 6 to the other end terminal of a center appear. tapped secondary winding I of a transformer I, The accompanying drawing illustrates an em- 40 the primary winding 1- of which is connected to bodlment of the invention. taps intermediate of the center tap and the end In the drawing, taps of the winding 4. A smoothing condenser Figure 1 is a diagram or a. system embodying 8 is connected across the electrodes 5 and 5 and the invention, while g a similar condenser 9 across the electrodes 6 Fig. 2 shows the relation of certain control and 6. variables of the system. An alternating biasing potential is impressed As is well understood, a, synchronous motor is upon the center tap of the winding l by the secusually started by connecting the armature to an ondary winding Ill of a transformer ill, the prialternating current source of energy, whereupon mary winding ill of which is connected to the the motor starts upas an induction motor. When bus bars Ll l and M2. One end terminal of the the motor armature has reached aspeed near winding Ni is connected to the center tap of synchronism, the field winding is supplied with winding 1', while the other terminal is connected direct current energy which causes the motor to to the cathode II"- of a gaseous electron tube ll, pull into step and run as a synchronous motor. which is also provided with an anode II and a In order not to cause too great a disturbance in control electrode ll. Connected between the cathode N and the control electrode is a resistor l2. The anode H is connected through a resistor l3 to one terminal of a condenser ll, the other terminal of which is connected to the center tap of winding 1. The condenser II is shunted by a parallel circuit comprising a gaseous electron tube l5, which is provided with a cathode l, an anode I5 and a control electrode 5, and a resistor I8 connected in series with the tube l5 between the cathode l5 and the common terminal of the condenser l4 and resistor II. The control electrode I5 is connected to one terminal of the secondary winding [1 of a transformer H, the primary winding of which is connected in series with a resistor l8 across the terminals of the winding 3'.

The system further provides for an adjustable direct current bias potential for the control electrodes 5 and 6. This direct current potential is obtained from a transformer 20, the primary winding 2|) of Which is connected across the bus bars LII and L|2, while the end terminals of its center tapped secondary winding 20 are connected to the anodes I9 and HP, respectively, of a full wave rectifying tube I9, which is also provided with a cathode l9. The cathode I9 is connected to the center tap of the winding 20', through a pair of series connected adjustable voltage dividers 2| and 22 and through a condenser 23, which is connected in parallel with said voltage dividers. The center tap of the winding 4 is connected to the adjustable contact of the divider 2|, while the second terminal of the winding is connected to the adjustable contact of the divider 22. The common terminal of the resistors 2| and 22 is connected to the common terminal of the condenser 14 and the resistors l3 and |6.

The system may further be provided with an electromagnetic relay 24, having an energizing winding 24, and normally closed contacts 24'. The contacts 24 normally connect a resistor 25 across the terminals of the winding 3*, as will be explained hereinafter.

The system thus far described operates as follows: A voltage is impressed between the control electrodes of the tubes 5 and 5, and their corresponding anodes. The principal component of such voltage is the voltage induced in the secondary winding 1 of transformer I. The transformer I is so connected to the secondar winding 4 of the transformer 4 that, during the respective positive half-cycles of the tubes 5 and 6, the potential impressed upon their respective control electrodes is negative, so as to prevent conduction of current through the tubes. As a result no current is supplied to the winding 3 of the motor, However, an additional unidirectional potential is superposed upon the aforementioned alternating component, tending to make the control electrodes of the tubes 5 and 5 positive with respect to their respective cathodes. This potential is supplied by the condenser M. The condenser 14 is supplied with half-wave rectified current through a circuit from the secondary winding Ill", through the condenser ll, resistor l3, and. the tube bias supplied by the condenser I4 varies with the charge of the condenser l4. At the moment of closure of the switch 2, the condenser of course has no charge and therefore no direct current bias from that source is available.

A second direct current bias is supplied by the resistor 2 I. It will be noted that as soon as the switch 2 is closed, th rectifier l9 causes a direct The unidirectional.

- 4 current to flow through the voltage dividers 2| and 22, and a portion of the voltage drop of the voltage divider 2| is impressed between the anodes of the tubes 5 and B, through the connection of the movable contact of said voltage divider as has been explained. Normally the adjustment of the resistor 2| is suchthat the direct current bias produced thereby is insufficient to overcome the negative bias derived from the transformer winding 1', so that upon closure of the switch 2 no exciting current is permitted to flow from the transformer 4 through the tubes 5 and 6, respectively, to the winding 3".

The condenser H receives a charge during each alternate half-cycle,- that is, when the tube H is conducting, The successive charges add to raise the potential of the condenser, However, a voltage is induced in the transformer ll, which at the moment of closure of the switch 2 impresses upon the grid |5, a positive potential which causes the tube I5 to become conducting,

thereby discharging th condenser and keeping it discharged as long as tube 5 is conducting. Thereafter the tube II can resume again the charging as described.

As the motor 3 accelerates, the frequency of the current induced in the winding 3 decreases. This obviously decreases the frequency of the periods during which the tube l5 becomes conducting to discharge the condenser H, while consequently the duration of the intervals during which the condenser H is charged increases. Thus the intervals between the discharge periods are gradually lengthened, so that the total charge acquired by the condenser l4 during such inter.- vals increases. It follows that 'by suitabl proportioning of the constants of the charging and discharge circuits of the condenser I, the latter attains between discharges a maximum potential which increases with the decrease of the slip frequency of the voltage induced in the winding 3", Ultimately the maximum charge of the condenser H which adds to the direct current bias of the resistor 2| is such as to overcome the negative bias derived from the transformer 1, so that at the respective half-cycle the tubes 5 and 6 become conducting and a unidirectional energizing current is impressed upon the field winding 3 which will cause the motor 3 to synchronize. v

As has been pointed out the slip frequency at which the voltage of the condenser I4 is of sufficient magnitude to give a positive bias to the control electrodes 5 and 6, depends upon the adjustment of the resistor 2| which furnishes the supplemental unidirectional bias. The voltage of the condenser as a function of th slip frequency is illustrated in Fig. 2. From this it will be seen that it increases very rapidly with decreasing slip at low slip frequency, so that a very precise response of the apparatus near synchronism is obtained.

The relay 24 has a relatively high inductance so that the flux induced in its magnetic circuit at high frequency is relatively low and that it does not attract its armature to open its contacts 24 until the frequency of the alternating current is relatively low. The relay assures that the field winding ll" is closed by a discharge circuit through the contacts 24 and the resistor 25, until the slip frequency has decreasedto a sufficiently low value, so that the winding 3 acts during starting as a secondary winding in a well known manner.

It will be apparent that the biasing voltage sup- '5 plied through the voltage divider 2| may be supplied by any other source, such as, for instance, a battery. 'The same is true of the biasing voltage supplied to the control electrode l5 by the voltage divider 22.

The condensers 8 and 9 are for the purpose of absorbing transients in thegrid voltage of the respective tubes. In the same manner the condenser 23 smoothes out the ripples of the rectified unidirectional current supplied by the transformer 20 to the rectifier l9.

If the synchronous motor should fall out of step due to an overload or some other cause, the direct current excitation of the motor will .be stopped due to the decrease of the potential of the condenser l4 and the resulting stoppage of the conduction of the tubes 5 and 6, and as the cause of the disturbance is removed, the motor will again accelerate and will be synchronized in the manner aforedesoribed.

While in the foregoing the invention is shown as incorporated in a system for controlling the excitation of a synchronous motor it is to be understood that it may also be employed to control the operation of translating devices in response to a variable frequency or in response to.

the variation of the ratio between two frequencies.

I claim:

1. In a synchronous motor starter, the combination with a synchronous motor having an alternating current armature and a direct current field winding, a source of unidirectional current and means to supply alternating current to said armature to accelerate said motor and operate it at synchronous speed, of a condenser, mean for supplying energy to said condenser at a given rate for a given frequency of the alternating current supply to charge said condenser, means responsive to the frequency of the voltage induced in said field winding by the alternating current in said armature to periodically discharge said condenser, and means responsive, to a given potential of said condenser to connect said source of unidirectional current to said field winding, thereby to render response of said last named means dependent upon the frequency of said induced voltage.

2. In a synchronous motor'starter, the combination with a synchronous motor having an alternating current armature and a direct current field winding, a source of unidirectional current the-alternating current in said armature toperiodically discharge said condenser, and rectifying means connected between said alternating current source and said field winding and responsive to a given potential of said condenser to supply a unidirectional energizing current to said field winding, thereby to render response of said last named means dependent upon the ratio of said two frequencies. .f

4. In a synchronous motor starter, the combination with a synchronous motor having an alternating current armature and a direct current field winding, an alternating current source and means to connect said source to said armature winding to accelerate said motor and operate it at synchronous speed, of a condenser, electronic means for supplying rectified current from said source to said condenser at a given rate for a given frequency of said alternating current source tocharge said condenser, electronic means re-' sponsive to the frequency of the voltage induced in saidfield winding by the alternating current in said armature to periodically discharge said condenser, and electronic rectifying means connected between said source and said field winding and responsive to a'given potential of said condenser to supply unidirectional energizing current to said field winding, thereby to render response of said last named means dependent upon the ratio of said two frequencies.

5. In a frequency responsive system, a condenser, means responsive to a given potential of said condenser, means for periodically tendingto charge said condenser to a given potential, and means affording periodic discharge of said condenser, the periodicity of said discharge decreasin as a function of a decreasing frequency of the system, thereby to render response of said first mentioned means dependent upon said frequency.

6. In a system involving a frequency which may vary and a varying frequency functionally and means to supply alternating current to said armature to accelerate said motor and operate it at synchronous speed, of a condenser, means periodically tending to charge said condenser to a given potential at the frequency of said alternating current source, means responsive to the frequency of the voltage induced in said field Winding by the alternating current in said armature to periodically discharge said condenser, and

means responsive to a given potential of said condenser to connect said source of unidirectional current to said field winding, thereby to render response of said last named means dependent upon the ratio of said two frequencies.

3. In a synchronous motor starter, the combination with a synchronous motor having an alternating current armature, and a direct current field Winding, an alternating current source and means to connect said source to said armature to accelerate said motor and operate it at synchronous speed, of a condenser, means periodically tending to charge said condenser to a given potential at the frequency of said alternating current source, means responsive to the frequency related to the first frequency, a condenser, means responsive to a given potential of said condenser, means tending to charge said condenser to a given potential in a period varying with the first frequency, and means afiording periodic discharge of said condenser, the periodicity of said discharge varying as a function of the second frequency, thereby to render response of said first mentioned means dependent upon the ratio of said two frequencies.

7. In a system involving a frequency which may vary and a varying frequency functionally related to the first frequency, a condenser, means responsive to a given potential of said condenser, means for supplying to said condenser a number of substantially equal quantities of energy succeeding each other at a rate varying as a function of the first of said frequencies to charge said condenser, and mean affording periodic discharge of said condenser, the periodicity of said discharge varying as a function of the second of said frequencies, thereby to render response of said first mentioned means dependent upon the ratio of said two frequencies.

8. In a system for supplying energy to a translating circuit from an electric energy source, the combination of means to connect said translating circuit to said source, a condenser, means for supplying energy to said condenser at a given rate to charge the same, means to supply a voltage of variable frequency and responsive to afford periodic discharge of said condenser during aling circuit to said source, a condenser, means re-.

sponsive to a variable frequency and tending to charge said condenser to a given potential in a period varying with said frequency, means responsive to a periodically varying voltage of varying frequency to afford discharge of said condenser during alternate half-cycles of said voltage, and means responsive to a given potential of said condenser to connect said translating circuit to said energy source, thereby to render response of said last named means dependent upon the ratio of said two frequencies.

JAMES B. REEVES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,019,769 Poole Nov. 5, 1935 2,110,015 Fitzgerald Mar. 1, 1938 2,176,742 La Pierre Oct. 17, 1939 1,983,656 Brown Dec. 11, 1934 2,102,371

Miller Dec. 14, 1937

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